This invention relates to solder bonding of electronic components to metallized substrates, and in particular to a method of getting a controlled amount of solder at each bonding site.
In circuit fabrication, the bonding of components to a metallized substrate is often critically important. For example, in the manufacture of many types of hybrid integrated circuits, components such as soldered-on transistors, discrete chip capacitors, and ceramic chip carriers are soldered to ceramic substrates containing thick film metallization. Typically, this is accomplished by applying a solder paste to bonding pads on the substrate, bringing the component leads into contact with the solder pads, and then heating to reflow the solder and establish the bond. This procedure, although adequate, often resulted in electrical shorting due to the fact that solder particles tend to be ejected from the paste during the reflow step and migrate to other portions of the substrate.
Alternative procedures also involve some significant drawbacks. For example, the solder may be applied by dipping the component into a molten solder bath, followed by applying the leads of the component to the bonding pads and reflowing the solder. In this procedure, it is often difficult to apply a sufficient amount of solder to the leads. Another method is to apply solder preforms to each bonding pad on the substrate prior to applying the leads to the pads. This method allows placement of a sufficient amount of solder at each bonding site and avoids ejection of the solder during reflow since there is no agitation of an organic carrier as when a paste is used. However, this method would require extensive machine design and re-tooling to be practical for manufacturing a wide range of circuits.
It is therefore a primary object of the invention to provide an economical means of soldering electronic components to a metallized substrate which places a precise amount of solder at each bonding site and avoids problems of solder migration over the substrate.